PySnooper README

The project's README from github.com/cool-RR/PySnooper. As one markdown file: README.md.

PySnooper - Never use print for debugging again

PySnooper is a poor man's debugger. If you've used Bash, it's like set -x for Python, except it's fancier.

Your story: You're trying to figure out why your Python code isn't doing what you think it should be doing. You'd love to use a full-fledged debugger with breakpoints and watches, but you can't be bothered to set one up right now.

You want to know which lines are running and which aren't, and what the values of the local variables are.

Most people would use print lines, in strategic locations, some of them showing the values of variables.

PySnooper lets you do the same, except instead of carefully crafting the right print lines, you just add one decorator line to the function you're interested in. You'll get a play-by-play log of your function, including which lines ran and when, and exactly when local variables were changed.

What makes PySnooper stand out from all other code intelligence tools? You can use it in your shitty, sprawling enterprise codebase without having to do any setup. Just slap the decorator on, as shown below, and redirect the output to a dedicated log file by specifying its path as the first argument.

Example

We're writing a function that converts a number to binary, by returning a list of bits. Let's snoop on it by adding the @pysnooper.snoop() decorator:

import pysnooper

@pysnooper.snoop()
def number_to_bits(number):
    if number:
        bits = []
        while number:
            number, remainder = divmod(number, 2)
            bits.insert(0, remainder)
        return bits
    else:
        return [0]

number_to_bits(6)

The output to stderr is:

Or if you don't want to trace an entire function, you can wrap the relevant part in a with block:

import pysnooper
import random

def foo():
    lst = []
    for i in range(10):
        lst.append(random.randrange(1, 1000))

    with pysnooper.snoop():
        lower = min(lst)
        upper = max(lst)
        mid = (lower + upper) / 2
        print(lower, mid, upper)

foo()

which outputs something like:

New var:....... i = 9
New var:....... lst = [681, 267, 74, 832, 284, 678, ...]
09:37:35.881721 line        10         lower = min(lst)
New var:....... lower = 74
09:37:35.882137 line        11         upper = max(lst)
New var:....... upper = 832
09:37:35.882304 line        12         mid = (lower + upper) / 2
74 453.0 832
New var:....... mid = 453.0
09:37:35.882486 line        13         print(lower, mid, upper)
Elapsed time: 00:00:00.000344

Features

If stderr is not easily accessible for you, you can redirect the output to a file:

@pysnooper.snoop('/my/log/file.log')

You can also pass a stream or a callable instead, and they'll be used.

See values of some expressions that aren't local variables:

@pysnooper.snoop(watch=('foo.bar', 'self.x["whatever"]'))

Show snoop lines for functions that your function calls:

@pysnooper.snoop(depth=2)

See Advanced Usage for more options. <------

Installation with Pip

The best way to install PySnooper is with Pip:

$ pip install pysnooper

Other installation options

Conda with conda-forge channel:

$ conda install -c conda-forge pysnooper

Arch Linux:

$ yay -S python-pysnooper

Fedora Linux:

$ dnf install python3-pysnooper

Citing PySnooper

If you use PySnooper in academic work, please use this citation format:

@software{rachum2019pysnooper,
    title={PySnooper: Never use print for debugging again},
    author={Rachum, Ram and Hall, Alex and Yanokura, Iori and others},
    year={2019},
    month={jun},
    publisher={PyCon Israel},
    doi={10.5281/zenodo.10462459},
    url={https://github.com/cool-RR/PySnooper}
}

License

Copyright (c) 2019 Ram Rachum and collaborators, released under the MIT license.

Media Coverage

Hacker News thread and /r/Python Reddit thread (22 April 2019)

PySnooper (cool-RR/PySnooper)

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16.6K ·GitHub
PySnooper
What is this?PySnooper is a Python debugging tool that traces a function and logs every line it runs, along with local variables, like set -x in Bash. One decorator replaces a pile of hand-written print statements.
How is it different from a full debugger in my IDE?A full debugger needs breakpoints and watches set up, which you often can't be bothered to do for a quick bug. PySnooper is the shortcut: one decorator line instead of carefully crafted print statements.
How do I use it on my own code?Add the @pysnooper.snoop() decorator to the function you're curious about. If you only care about part of a function, wrap just that part in a with block instead of tracing the whole thing.
Will it cope with a big, messy production codebase?Yes — zero setup is its headline strength. You just slap the decorator on the misbehaving function, even inside a sprawling enterprise codebase.
What does the snoop output actually look like?The README demos a function that converts a number to binary into a list of bits. With the snoop decorator on it, stderr shows a play-by-play: each line as it runs and exactly when local variables change.
Can I cite PySnooper in academic work?Yes, the README provides a specific citation format for using PySnooper in academic publications.
What are similar projects?icecream, memray, vConsole, rr, debug, loguru, delve, gdbgui
Which topics is it in?debug, tracing, debugger, logging, python-library, debugging, free, tool, library, python, cross-platform, developer-tools
Creator
cool-RR
Size
0.4 MB
Stars
16.6K
Updated
CommitJun 08Release1.2.0Jan 05
Age
7y
Commits · 90 Days